There are very interesting workflows on our way. Start with
an image, produce a path in GIMP, save as SVG, open in
INKSCAPE, adjust, save as EPS, convert to
MetaPost with PSTOEDIT, edit the code,
compile, convert to raster with
GHOSTSCRIPT (gs -q -sDEVICE=pngalpha -r225 -dNOPAUSE
-sOutputFile=outputimage.png -dTextAlphaBits=4
-dGraphicsAlphaBits=4 metapostoutput.eps </dev/null)
and edit with ImageMagick.